1. Field of the Invention
This invention concerns large area liquid crystal displays. Such displays are useful as televisions, notice boards, or maps in public areas to give information readable at distances of many meters. Also such displays are useful in large area high definition television receivers.
2. Discussion of Prior Art
Liquid crystal displays are well known devices which rely on an electric field induced change of molecular ordering in liquid crystal materials. In one known type of device a layer of liquid crystal material is contained between two sheets of glass. Transparent electrodes are coated on the inner faces of these sheets and are used to apply an electric field across the liquid crystal layer. Upon application of a suitable electric field the liquid crystal molecules change their alignment, and this gives rise to a visible change. There are many types of devices using this effect, for example a twisted nematic, a phase change, etc. To increase the number of bits of information that can be displayed it is common to form the electrodes into row and columns making an x,y matrix of electrode intersections, termed a pixel of information. By applying an appropriate voltage to a row and a column each intersection can be switched in turn. The known technique for addressing such a matrix is multiplex addressing. This involves indexing a WRITE pulses onto each row in turn whilst appropriate DATA voltages (eg plus or minus DATA) are applied to each column electrode. The display is continually refreshed by repeating the addressing sequence.
A disadvantage of devices using glass sheets is the precise control of liquid crystal layer thickness and the filling of the small, typically 8 .mu.m, space between sheets.
These problems have to some extent been overcome by encapsulating liquid crystal droplets in a polymer matrix and forming the result into sheets of controlled thickness. Such sheets are variously known as Nematic Curvilinear Aligned Phase (NCAP), Polymer Dispersed Liquid Crystal (PDLC), Polymer Networked Liquid Crystal (PNLC), and are described for example in Mol. Cryst. Liq Cryst. Inc. Nonlin. Opt. (1988) 157 pages 427-441, Liquid crystal Crystals (1988) 3(11) 1543-1559, U.S. Pat. No. 4,435.047, U.S. Pat. No. 4,688,900, French Patent 72,17274, and Application GB 89 28,282.6. These polymer sheets can be enclosed between cell walls carrying electrodes and addressed as for the conventional cells described above.
A problem common to the conventional glass walled liquid crystal cell and those using sheets of polymer encapsulated liquid crystal is that of addressing very large displays. The term large applies both to the physical size and to the amount of information displayed. Glass walled cells are difficult to manufacture in physically large sizes. Both these and those cells using polymer encapsulated liquid crystal material are difficult to address when the number of separately addressable pixels becomes large. The reason for this is that each pixel needs to be re addressed during each frame time. A frame time is the time taken to address the whole display once; the display is continually addressed one frame after another. The reason each pixel needs repeated addressing is that the liquid crystal molecules can relax back to an OFF state a relatively short time after receiving a voltage turning them into an ON state: i.e. the materials are monostable.
Two approaches have been used to overcome this relaxing back to an OFF state. One uses chiral smectic liquid crystal materials in a device which is bistable, ie the material remains in one state arbitrarily defined as OFF or ON once switched. The other approach uses non linear elements associated with each addressable pixel. With these displays one cell wall is formed as a sheet ground electrode and pixels are defined by shaped electrode patches on the other cell wall. Between the pixel patches are source and gate bus lines with a non linear device at each bus line intersection associated with each pixel patch. Multiplex addressing is used as above but the voltage waveforms and timing is different. Many high quality displays have been produced using switches at each pixel, these have been used for television type displays.
The non linear elements are formed by conventional photolithographic techniques on glass walls used for liquid crystal cells. As with all manufacturing techniques for making various solid state devices, eg field effect transistors, etc, not all devices produced on a large sheet will work correctly. This is termed the yield; it is difficult to make a large sheet with every device a working device. For a satisfactory display each non linear element must work correctly. The larger the number of switches on a substrate the greater the chance of one or more switch not working, i.e. the yield is lower. Thus although it may be theoretically possible to produce a very large number of switches on a substrate, and hence produce a very large liquid crystal display, it is not practical to produce large displays with a commercially useful yield.
One way of overcoming this problem is to use a number of separate and independent displays and join them together. This has been done with television screens with each screen arranged to display a portion of a whole scene. Unfortunately the joins between screens spoils the display.
As an alternative to television screens patent GB 1,522,520 describes a matrix of independent liquid crystal cells each separately addressable to collectively display information. The apparent gap between each cells is bridged by a fibre optical wedge so that from a distance the gap between each cell appears to be non existent. In practice this arrangement was bulky and difficult to manufacture.